1. Field of the Invention
The present invention relates generally to diffusion coatings such as chromizing for corrosion resistance and particularly to the production of such coatings by induction heating same during the coating process.
2. Description of the Related Art
Diffusion coating are frequently applied on the surfaces of various high-temperature components to enhance their corrosion resistance. The coatings are achieved by diffusing reactive elements, such as Cr, Si, Al, and rare-earth elements, individually or simultaneously, into the component surface at elevated temperatures. Upon exposure to corrosive environments, these coatings can provide enhanced corrosion protection on the component surfaces by forming more protective oxides or improving the oxide integrities. Currently, three processing techniques are used: 1)pack cementation, 2)slurry, and 3)blanket processes.
A typical pack cementation process involves burying the parts to be coated with a pack mix in a retort. The pack mix consists of powders of a source metal or alloy (masteralloy), a small amount of halide salt (activator), and a large amount of inert oxide (filler). The retort is heated to the coating temperature in a furnace and held therein for an extended period of time. An inert cover gas is generally passed through the retort to maintain a reducing condition during the coating process. The retort is heated inside a high-temperature furnace which is either electric for laboratory and bench-scale productions or gas-fired for large-scale commercial productions.
Compared to pack cementation, the slurry and blanket processes require some modifications in the physical arrangement of the pack mix. In the slurry process, a layer of the pack mix is placed onto the surfaces of the substrates to be coated by water-base slurry spray or dipping; whereas in the blanket process, the mix is first accommodated in a ceramic fiber cloth via water-base slurry spray. The ceramic cloth is then dried and placed next to the substrate surfaces. Other than these modifications, the coating mechanisms involved in the slurry and blanket processes are identical to those in pack cementation.
All of these coating processes share a common drawback. The substrates are separated from the heat source of the electric or gas-fired furnace by a thick layer of ceramic powder filler or fiber cloth. The thermal conductivities of these ceramic materials are extremely low and therefore, they act as thermal insulators. As a result, the heating time required for raising the substrate temperature from room temperature to the coating temperature, as well as the cooling time from the coating temperature to room temperature, are significantly lengthy. The prolonged heating and cooling time attributes to excessive energy consumption, slow production rates, and unnecessary labor hours. As a result, the production cost for diffusion coatings is elevated.